A wireless network typically includes access points (e.g. base stations) through which User Equipment (UE) may access the wireless network. Each access point typically services a softly delineated geographic area that is known as a coverage area, in which UE can be used to establish a wireless link with the particular access point. In other words, within a coverage area corresponding to an access point UE can typically expect to be able to communicate (transmit and receive signals) wirelessly with the corresponding access point.
In general, transmissions sent to an access point originating from one or more UE's are collectively known as an uplink (to the access point). This is an example of a many-to-one communication system in which multiple UE's must share access to a common wireless channel. It is difficult to manage multiple-user access to a common wireless channel since respective transmissions originating from different UE's cannot easily be synchronized in practical circumstances. Specifically, in a cellular network, an uplink consists of many point-to-point transmissions that are all directed to a base station (access point) and that originate from respective UE's operating within a cell (coverage area) serviced by the base station.
An access scheme, commonly known as an uplink air interface, must be specified and followed to control the way each UE within a wireless communication network transmits signals to access points (e.g. base stations) so that the common wireless channel is effectively shared by multiple UE's. In cellular networks the uplink air interface must take into account transmissions from multiple UE's operating in the same cell as well as transmissions from UE's operating in adjacent cells. In other words, for wireless communications to be effective a method of dividing the common wireless channel, otherwise known as channelization, must be applied so that each UE can gain transmission access to some portion of the common wireless channel for some reasonable amount of time.
Different multiple-user access schemes have been developed and employed in cellular networks for the uplink air interface. Examples of such multiple-user access schemes include channelization based on: i) frequency division; ii) time division; and iii) code division. According to Frequency Division Multiple Access (FDMA) the common wireless channel is divided into sub-channels, each of which can be dedicated to a single UE. On the other hand, basic Time Division Multiple Access (TDMA) allows multiple users to transmit into the entire common wireless channel one at a time. Code Division Multiple Access (CDMA) allows multiple UE's to transmit into the entire common wireless channel simultaneously by respectively assigning each UE a unique spreading code (cover) that is orthogonal to all other spreading codes assigned to other UE's. In other words, the spreading codes (cover) serve as identifiers or covers that are included in each of the UE's respective transmissions.
The maximum data rate associated with uplink transmission for each of the aforementioned schemes is limited. For example, in 3G (i.e. third generation) cellular networks, based on CDMA, the multiple-access interference inherent to CDMA limits the data rate transmission to 2 Mbps. Moreover, orthogonality between the transmissions from different UE's, provided by the respectively assigned spreading codes, is difficult to maintain since the different UE's do not typically transmit signals synchronously. Once the orthogonality between the transmissions from the different UE's is compromised multiple-access interference is introduced, and this limits the maximum uplink data rate. Generally, in cellular networks the total multiple-access interference can be made up of intra-cell and inter-cell multiple-access interferences.
European digital audio broadcast services and some WLAN (Wireless Local Area Network) uplink access schemes employ a modulation technique known as Orthogonal Frequency Division Modulation (OFDM). OFDM also lends itself to digital television, and is being considered as a method of obtaining high-speed digital data transmission over conventional telephone lines. Advantageously, OFDM allows for simple processing to combat dispersive channel distortions and high speed data rate transmission in broadcast environments and single point-to-point communications. The drawback to OFDM is that it does not inherently provide for multiple-user access despite being very effective for broadcast and single point-to-point communications.
OFDM has been combined with Time Division Multiplexing (TDM) in systems that require multiple-user access. For example, in some WLAN networks OFDM is combined with TDM to provide the multiple access capabilities. Namely, OFDM is used for uplink transmissions from one user at a time, with multiple-user access being arranged in a TDM fashion. However, this type of uplink access scheme cannot effectively support cellular network deployment and mobility because it does not provide the quality of service and features required in cellular networks. In addition, these schemes do not support circuitry data services such as voice.